1. Field of the Invention
The present invention relates to a method for growing a silicon single crystal that is a material of silicon wafers, and more particularly, to a method for growing a silicon single crystal, which is capable of inhibiting dislocation caused by a thermal stress and enables the growth of single crystals having dislocation-free portions at satisfactory yield.
Priority is claimed on Japanese Patent Application No. 2005-179995, filed Jun. 20, 2005, and U.S. Provisional Patent Application No. 60/693,944 filed Jun. 27, 2005 the contents of which is incorporated herein by reference.
2. Description of Related Art
The Czochralski method (CZ method) is a well known crystal growth method for producing a silicon single crystal as a raw material of silicon wafers. For an effective production of silicon single crystals having desired quality with the CZ method, controlling of temperature of a silicon single crystal during the crystal growth has been known in the prior art. For example, a technique has been proposed for increasing the maximum pulling rate by quenching a vicinity of a solid-liquid interface of a silicon single crystal during the crystal pulling (see, for example, Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H11-199385).
However, in the technique described in Patent Document 1, dislocations are easily generated by thermal stress caused by cooling of the silicon single crystal, resulting in a problem on poor productivity and yield.
Generally, when dislocations occur in a silicon single crystal during crystal pulling, the pulled-up silicon single crystal is molten and is pulled again to obtain a silicon single crystal having a long dislocation-free portion.
However, repeated pulling and melting of a silicon single crystal require long time for pulling, and therefore decrease productivity of crystals. If the growth of dislocation-free crystal is abandoned and the crystal growth is terminated, a large amount of silicon melt remains in the crucible. That means a waste of raw materials. In addition, volumetric expansion in the solidification process of the remaining silicon melt causes a possibility of destruction of a crucible and heater. Therefore, when dislocations occur multiple times, and pulling of a silicon single crystal having a long dislocation-free portion can not be expected even by melting the pulled-up crystal and pulling it again, a silicon single crystal generating dislocations is pulled as it is.
A silicon single crystal generating dislocation multiple times has large shear in the crystal grain, which is caused by thermal stress during the crystal growth. Therefore, after cooling the pulled-up crystal to room temperature, a large residual stress occurs due to the shear of the crystal grain. Consequently, there has been a problem of cracking of the crystal by a stress equal to or greater than the yield stress of silicon easily caused by a slight impact, for example, by an impact during taking the pulled-up single crystal out of a furnace or transporting the silicon single crystal taken out of a furnace.
In consideration of the aforementioned circumstances, the present invention provides a method for growing a silicon single crystal, which has superior productivity and is capable of inhibiting occurrence of dislocations caused by a thermal stress generated by controlling the temperature on the side surface of the silicon single crystal during crystal growth. The method allows to grow, at satisfactory yield, silicon single crystals being resistant to cracking and having long dislocation-free portions.